Multi-stage cryocooler with concentric second stage

ABSTRACT

A multi-stage cryocooler includes a concentric second-stage pulse tube expander in which a pulse tube is located within a second-stage regenerator. In one embodiment, an inner wall of the regenerator also functions as an outer wall of the pulse tube. In another embodiment, there is an annular gap between an inner wall of the regenerator and an outer wall of the pulse tube. The gap may be maintained at a low pressure, approaching a vacuum, by placing the gap in fluid communication with an environment around the cryocooler, such as the low-pressure environment of space. The integrated second-stage structure, with the pulse tube within the annular regenerator, provides several potential advantages over prior multi-stage cryocooler systems.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention is in the field of cryocoolers, and more particularly inthe field of regenerative cryocoolers.

2. Background of the Related Art

Multi-stage cryocoolers are of fundamental interest for manyapplications in which cryogenic cooling is required. For example, someapplications require the simultaneous cooling of two objects tocryogenic, but different, temperatures. In the case of a long waveinfrared sensor, for instance, the focal plane assembly may require anoperating temperature of around 40 K, while the optics may need to bemaintained at a different temperature, such as about 100 K. One approachfor such situations is to use a single-stage cooler and extract all ofthe refrigeration at the coldest temperature. However, this isthermodynamically inefficient. Another approach is to use twosingle-stage cryocoolers with one each at the two temperaturereservoirs. This approach has the disadvantage of being expensive andlarge in size. A better approach that has been done in the past is touse a two-stage cryocooler with the first-stage cooling the higheroperating temperature component, and the second stage cooling the loweroperating temperature component. Multi-stage cryocoolers are generallymore efficient than single-stage coolers, because a portion of theinternal parasitic thermal losses can be removed from the system athigher temperatures, thus producing less entropy generation.

FIG. 1 shows a portion of a prior art cryocooler 10. The cryocooler 10includes a compressor 11 that is coupled to a first-stage Stirlingexpander 20 with a first-stage regenerator 21, a plenum 22, and a pistonor displacer 23. The piston 23, which contains the regenerator 21,oscillates within a cold cylinder 25. A wall of the cold cylinder 25provides first stage pressure containment and thermal isolation from theambient warm end. The plenum 22 and a motor assembly 27 are containedwithin an expander housing 26. The first-stage expander 20 also includesa first-stage heat exchanger 24 in a first-stage manifold 28. The pistonor displacer 23 is used to expand the working gas, such as helium,downstream of the regenerator 21 such that refrigeration is produced inthe first-stage heat exchanger 24. The working gas absorbs the firststage heat load from the environment as it passes through thefirst-stage heat exchanger 24. The first-stage heat exchanger 24 is inpneumatic communication with a second-stage pulse tube expander 30,where the (colder) second-stage refrigeration is produced. The pulsetube expander 30 includes a second-stage regenerator 31 and a pulse tube32. The second-stage regenerator 31 and the pulse tube 32 may begenerally parallel to one another, forming legs of a U-shapedconfiguration. The second-stage regenerator 31 and the pulse tube 32 arelinked together by a flow passage 36 in a second-stage manifold 41. Theflow passage 36 links a downstream end of the second-stage regenerator31 with an upstream end of the pulse tube 32. End caps 42 and 43 closeoff the respective ends of the second-stage regenerator 31 and the pulsetube 32, within the second-stage manifold 41. A second-stage cold heatexchanger 44 is at an upstream end of the pulse tube 32, in thesecond-stage manifold 41. A second-stage warm heat exchanger 46 is at adownstream end of the pulse tube 32, in the first-stage manifold 28. Thecryocooler 10 may be used to cool objects thermally coupled to either orboth of the manifolds 28 and 41. Objects in thermal communication withthe first-stage manifold 28 are cooled at a first cold temperature, andobjects in communication with the second-stage manifold 41 are cooled atan even lower cold temperature. Further details regarding prior artcryocoolers may be found in commonly-assigned U.S. Pat. Nos. 6,167,707,and 6,330,800, the descriptions and figures of which are incorporatedherein by reference.

In installation of the prior art cryocooler 10, the cold cylinder 25,the first-stage manifold 28, and the second-stage pulse tube expander 30(collectively a cold head 50) are often required to be supported only atthe expander housing 26. This leaves the second-stage pulse tubeexpander 30, the second-stage manifold 41, the first-stage manifold 28,and much of the cold cylinder 25, cantilevered off of the housing 26.This has caused difficulties, particularly in space flight applications,where the cooling system must be able to withstand loads and randomvibrations generated during launch.

From the foregoing it will be appreciated that improvements inmulti-stage cryocoolers may be possible.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a multi-stage cryocoolerincludes: a first-stage expander; and a second-stage pulse tube expanderdownstream of the first-stage expander. The second-stage expanderincludes an annular second-stage regenerator.

According to another aspect of the invention, a multi-stage cryocoolerincludes: a first-stage Stirling expander; and a second-stage pulse tubeexpander downstream of the first-stage expander. The second-stageexpander includes: a second-stage regenerator; and a pulse tube withinand radially surrounded by the second-stage regenerator.

According to yet another aspect of the invention, a multi-stagecryocooler includes: a first-stage Stirling expander; and a second-stagepulse tube expander downstream of the first-stage expander. Thefirst-stage expander includes a first-stage manifold. The second-stageexpander includes: an annular second-stage regenerator; a pulse tubeconcentrically within the second-stage regenerator; and a second-stagemanifold. The first-stage manifold is coupled to an upstream end of thesecond-stage regenerator, and to a downstream end of the pulse tube. Thesecond-stage manifold is coupled to a downstream end of the second-stageregenerator, and to an upstream end of the pulse tube. The second-stageregenerator, the pulse tube, and the second-stage manifold are allsubstantially axisymmetric.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative embodiments of theinvention. These embodiments are indicative, however, of but a few ofthe various ways in which the principles of the invention may beemployed. Other objects, advantages and novel features of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings, which are not necessarily to scale:

FIG. 1 is a cross-sectional view of a prior art multi-stage cryocooler;

FIG. 2 is a cross-sectional side view of a multi-stage cryocooler inaccordance with the present invention;

FIG. 3 is a cross-sectional view of one embodiment of the second stageof the cryocooler of FIG. 2;

FIG. 4 is a cross-sectional view of another embodiment second stage ofthe cryocooler of FIG. 2;

FIG. 5 is a detailed view of a portion 5-5 of the second stage of FIG.4; and

FIG. 6 is a cross-sectional view of an alternate embodiment cryocoolerin accordance with the present invention, having an angled second stage.

DETAILED DESCRIPTION

A multi-stage cryocooler includes a concentric second-stage pulse tubeexpander in which a pulse tube is located within a second-stageregenerator. In one embodiment, an inner wall of the regenerator alsofunctions as an outer wall of the pulse tube. In another embodiment,there is an annular gap between an inner wall of the regenerator and anouter wall of the pulse tube. The gap may be maintained at a lowpressure, approaching a vacuum, by placing the gap in fluidcommunication with an environment around the cryocooler, such as thelow-pressure environment of space. The integrated second-stagestructure, with the pulse tube within the annular regenerator, providesseveral potential advantages over prior multi-stage cryocooler systems.First, the mass of the first- and second-stage manifolds may be reducedbecause of the placement of the pulse tube within the second-stageregenerator. The second-stage manifold is used for putting theregenerator and the pulse tube in communication with one another, andfor allowing thermal coupling to heat loads. This may reduce mechanicalloads on the cold cylinder, which may be mechanically supported only atone end (the end opposite the first-stage manifold). The axisymmetricconfiguration of the second-stage expander facilitates configuring thesecond-stage manifold axisymmetrically, allowing substantially isotropicload carrying characteristics, and potentially simplifying integrationfor an end user, who need not constrain orientation of thermal strapsrelative to the second-stage manifold. Further, the placement of thepulse tube within the second-stage regenerator may allow for moreuniform flow from the second-stage regenerator through the second-stagemanifold to the pulse tube. For instance, the pulse tube may be locatedaxisymmetrically within the second-stage regenerator, and the manifoldmay be configured to allow substantially axisymmetric flow into anupstream end of the pulse tube. Finally, the integration of thesecond-stage regenerator and the pulse tube into a single contained unitmay also increase the structural strength of the second-stage pulse tubeexpander.

With reference initially to FIG. 2, details are now discussed of amulti-stage cryocooler 100. The cooler 100 includes a compressor 110coupled to a first-stage expander 120, such as a Stirling expander. Theexpander 120 may be substantially identical to the expander 20 of theprior art cryocooler 10 (FIG. 1), and may include such parts as afirst-stage regenerator 121, a plenum 122, and a piston or displacer123, a cold cylinder 125, an expander housing 126, and a motor assembly127. Working fluid exiting the first-stage regenerator 121 proceeds intoa first-stage heat exchanger 124 that is in a first-stage manifold 128.The first-stage heat exchanger 124 includes through holes proceedingthrough the first-stage manifold 128, for allowing flow of the workingfluid into a second-stage pulse tube expander 130. The first-stagemanifold 128 may be maintained at a first-stage cold temperature, andmay be linked to heat-producing items via suitable thermal straps (notshown) to cool or maintain temperature of the heat-producing items.

The cold cylinder 125 (and its contents) and the second-stage pulse-tubeexpander 130 are parts of a cold head 129. The cold head 129 ismechanically coupled to the expander housing 126.

The second-stage pulse tube expander 130 includes a second-stageregenerator 131, a pulse tube 132, and a second-stage manifold 134. Theworking gas proceeds from the first-stage manifold 128 into thesecond-stage regenerator 131. Within the second-stage manifold 134, theworking gas is ported into the pulse tube 132. It flows through thepulse tube 132, and into the first-stage manifold 128. From thefirst-stage manifold 128, the outlet from the pulse tube 132 may becoupled to a surge volume 136, via an inertance port 138. The surgevolume 136 may be maintained at an ambient warm temperature. Furtherdetails regarding configuration and use of an ambient-temperature surgevolume may be found in commonly-assigned U.S. application Ser. No.10/762,867, titled “Cryocooler With Ambient Temperature Surge Volume”filed Jan. 22, 2004, the description and figures of which areincorporated herein by reference.

The pulse tube 132 is located radially within the second-stageregenerator 131. The second-stage regenerator may be an annularregenerator, with the pulse tube 132 centered within the second-stageregenerator 131. The pulse tube 132 has a second-stage cold heatexchanger 141 located at an upstream end 142 of the pulse tube 132,within the second-stage manifold 134. The pulse tube 132 also has asecond-stage warm heat exchanger 143 located at a downstream end 144 ofthe pulse tube 132, within the first-stage manifold 128. Thesecond-stage cold heat exchanger transfers heat from the second-stagemanifold 134, which may be made of a suitable material, such as copper.The second-stage warm heat exchanger 143 transfers heat to thefirst-stage manifold 128.

The second-stage expander 130 may be substantially axisymmetric, withthe pulse tube 132 being axisymmetrically located within thesecond-stage regenerator 131. The first-stage manifold 128 and thesecond-stage manifold 134 may also be substantially axisymmetric. Thestructural load bearing capability of the both expander stages may thusbe substantially independent of the radial orientation of any structuralloading force. Thus there advantageously may be no need to take intoaccount orientation of the second-stage expander 130 when thermallycoupling the second-stage manifold 134 to devices to be cooled, by useof cryogenic thermal straps (not shown). By contrast, in the U-turnsecond-stage configuration, such as shown in the second-stage expander30 (FIG. 1), a designer must take into account variations in structuralstrength for different orientations, when attaching loads to thesecond-stage manifold 41 (FIG. 1).

Perhaps more importantly, the axisymmetric cold head 129, with itsaxisymmetric second-stage expander 130, may advantageously increase thefrequency of the lowest cantilever bending mode. An embodiment of theconfiguration described herein has been found to have a fundamentalcantilever bending mode frequency above 200 Hz. This compares with priordesigns having lowest cantilever bending modes between 115 and 160 Hz.Since deflection is reduced as the inverse square of the frequency, thehigher natural frequency of the cold head 129 greatly reduces itssensitivity to vibrations.

Another advantage of the axisymmetric second-stage expander 130 is thatflow may be substantially axisymmetric in both the second-stageregenerator 131 and the pulse tube 132. The flowing working gas may beintroduced substantially axisymmetrically at an upstream end 152 of thesecond-stage regenerator 131, where the regenerator 131 interfaces withthe first-stage manifold 128. In the second-stage manifold 134 flow ofthe working gas may be substantially axisymmetrically turned from adownstream end 154 of the second-stage regenerator 131, into theupstream end 142 of the pulse tube 132. The substantial axisymmetry inflow within the second-stage regenerator 131 and the pulse tube 132 mayresult in more uniform performance, and thus improved performance,relative to prior cryocoolers with non-uniform flow. This increaseduniformity in performance may be due to decreased mixing at the pulsetube cold end.

Turning now to FIG. 3, certain details are shown of one embodiment ofthe second-stage expander 130. The embodiment shown in FIG. 3 is atwo-tube embodiment, with an interior wall 160 serving as both the outerwall of the pulse tube 132, and as the inner wall of the second-stageregenerator 131. A second tube or wall 162 serves as the outer wall ofthe second-stage regenerator 131.

The second-stage manifold 134 has longitudinal flow passages 170 and172, and radial flow passages 174 and 176. The longitudinal flowpassages 170 and 172 may be parts of an annular gap between an innerportion 180 and an outer portion 182 of the second-stage manifold 134.The radial flow passages 174 and 176 may be portions of a disk-shapedflow cavity beneath an end cap 186 of the first-stage manifold 134. Flowmay proceed from the downstream end 154 of the second-stage regenerator131, through the longitudinal flow passages 170 and 172 through theradial flow passages 174 and 176, and into the second-stage cold heatexchanger 141 at the upstream end 142 of the pulse tube 132. Thisturning of the flow from the downstream end 154 of the second-stageregenerator 131, to the upstream end 142 of the pulse tube 132, may besubstantially axisymmetric. Alternatively, flow passages within thesecond-stage manifold 134 may allow for some asymmetry in turning of theflow from the second-stage regenerator 131 to the pulse tube 132.

FIG. 4 shows an alternate embodiment of the second-stage expander 130, athree-tube embodiment that includes an insulator 190 between an innerwall 192 of the regenerator 131, and an outer wall 194 of the pulse tube132. The insulator 190 may be a gap 196 between the walls 192 and 194.The gap 196 may be a vacuum gap, for instance, having a pressure withinthe gap 196 of about 1×10⁻⁵ torr or less. As shown, the gap 196 may be arecess formed by a thinned portion 199 of the pulse tube wall 194.Alternatively, the gap 196 may be formed by other suitable methods.

The gap 196 may be in communication with an ambient environment aroundthe cryocooler 100. The first-stage manifold 128 may have ports 200 and201 to allow the gap 196 to be in fluid communication with theenvironment surrounding the cryocooler 100. Since cryocoolers aretypically utilized in vacuum environments, such as the vacuum of space,placing the gap 196 in communication with the environment surroundingthe cryocooler 100, and allowing the gap 196 to be at a low-pressurevacuum.

The gap 196 may have a width or thickness on the order of 10 mils. Thegap 196 may have any suitable width such that sufficient vacuumconductance exists to pull a hard vacuum in the entire gap 196, via theports 200 and 201. The gap 196 may be an annular gap, or may have othersuitable shapes.

With reference now in addition to FIG. 5, the regenerator inner wall 192and the pulse tube wall 194 may have respective low-radiative-emissivitysurfaces 202 and 204, facing the gap 196. The low-radiative-emissivitysurfaces may be configured to minimize radiative heat transfer acrossthe gap 196. The low-radiative-emissivity surfaces 202 and 204 may begold-plated surfaces, or may be polished-metal surfaces, such assurfaces of polished stainless steel.

It may be advantageous to have the vacuum gap 196 between the pulse tube132 and the second-stage regenerator 131 to prevent undesired heattransfer between the pulse tube 132 and the second-stage regenerator131, which otherwise may degrade performance of the second-stageexpander 130. The temperature gradients along the second-stageregenerator 131 and the pulse tube 132 are different from oneanother—the temperature gradient along the second-stage regenerator 131is nearly linear, while the temperature gradient along the pulse tube132 is non-linear. Without insulation between the second-stageregenerator 131 and the pulse tube 132, a radial heat flow would occurbetween the two devices, possibly degrading device performance. Puttinga vacuum gap between the devices minimizes the radial heat transfer, andthus may improve performance.

Nevertheless, the radial heat transfer described in the previousparagraphs may be acceptable in some situations, and the two-tubeconfiguration of FIG. 3 may be suitable for those situations. Forexample, for a 1-Watt, 77-Kelvin cryocooler the two-tube configurationmay be suitable, with some level of radial heat transfer between thesecond-stage regenerator 131 and the pulse tube 132 being tolerated. Butfor a cryocooler operating at a lower temperature, for example 10Kelvin, the radial heat transfer may significantly affect operation, andthe three-tube configuration of FIGS. 4 and 5 may be preferable.

With reference to FIG. 6, the second-stage expander 130 may be angledwith regard to the first-stage expander 120. The term “angled” as usedherein, refers to a non-zero angle between the second-stage expander 130and the first-stage expander 120, such that the second-stage expander130 is not in line with the first-stage expander 120. As shown in FIG.6, the second-stage expander 130 may be at a 45° angle relative to thefirst-stage expander 120. More broadly, it may be advantageous to orientthe second-stage expander 130 at any of a wide variety of anglesrelative to the first-stage expander 120, such as angles of 45°, 90°, orany other suitable angles.

The various embodiments of the cryocooler 100 described here allow forimproved structural characteristics of the cold head 129. In addition,heat transfer performance of the second-stage expander 130 may beimproved by providing more uniform, substantially axisymmetric, flow. Itwill be appreciated that the improved structural and heat transferperformance may allow for cryocoolers with decreased cost and weight aswell.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described elements (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such elements are intended to correspond,unless otherwise indicated, to any element which performs the specifiedfunction of the described element (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one or more of several illustrated embodiments, suchfeature may be combined with one or more other features of the otherembodiments, as may be desired and advantageous for any given orparticular application.

1. A multi-stage cryocooler comprising: a first-stage expander; and asecond-stage pulse tube expander downstream of the first-stage expander;wherein the second-stage expander includes (i) an annular second-stageregenerator with an inner wall and (ii) a pulse tube, with an outerwall, substantially centered radially within the second-stageregenerator, wherein the second-stage regenerator inner wall and thepulse tube outer wall are separated by a gap.
 2. The cryocooler of claim1, wherein the gap is a substantially annular gap.
 3. The cryocooler ofclaim 1, wherein the gap is in fluid communication with an environmentaround the cryocooler.
 4. The cryocooler of claim 1, wherein respectivesurfaces of the second-stage regenerator inner wall and the pulse tubeouter wall that face the gap are low-radiative-emissivity surfaces. 5.The cryocooler of claim 4, wherein the low-radiative-emissivity surfacesare gold plated surfaces.
 6. The cryocooler of claim 4, wherein thelow-radiative-emissivity surfaces are polished metal surfaces.
 7. Thecryocooler of claim 1, wherein the gap is a vacuum gap maintained at apressure of 1×10⁻⁵ torr or less.
 8. The cryocooler of claim 1, whereinthe gap has a thickness on the order of 10 mils.
 9. The cryocooler ofclaim 1, wherein the second-stage expander further includes asecond-stage manifold mechanically coupled to a downstream end of thesecond-stage regenerator, and mechanically coupled to an upstream end ofthe pulse tube; and wherein the second-stage regenerator, the pulsetube, and the second-stage manifold are all substantially axisymmetric.10. The cryocooler of claim 1, wherein the second-stage pulse-tubeexpander is angled relative to the first-stage expander.
 11. Amulti-stage cryocooler comprising: a first-stage Stirling expander; anda second-stage pulse tube expander downstream of the first-stageexpander; wherein the second-stage expander includes: a second-stageregenerator; a pulse tube within and radially surrounded by thesecond-stage regenerator; and a pap between the second-stage regeneratorand the pulse tube.
 12. The cryocooler of claim 11, wherein the gap is avacuum gap maintained at a pressure of 1×10⁻⁵ torr or less.
 13. Thecryocooler of claim 11, wherein low-radiative-emissivity surfaces of theregenerator and the pulse tube adjoin the gap.
 14. The cryocooler ofclaim 13, wherein the low-radiative-emissivity surfaces are gold platedsurfaces.
 15. The cryocooler of claim 13, wherein thelow-radiative-emissivity surfaces are polished metal surfaces.